Construction and validation of a nomogram prediction model for chronic low back pain after PKP for lumbar compression fractures

The aim was to study the independent risk factors for chronic low back pain after lumbar compression fractures undergoing percutaneous kyphoplasty (PKP), and to establish a nomogram prediction model accordingly. Data were collected from patients with lumbar compression fractures from January 2017 to December 2021 at the Affiliated Hospital of Xinjiang Medical University. Univariate and multivariate logistic regression analyses were used to determine the independent risk factors for chronic low back pain after receiving PKP for lumbar compression fractures, and the corresponding nomogram was established. Receiver operating characteristic (ROC) curves were plotted and area under the curve (AUC) was calculated, and calibration curves and decision curve analysis (DCA) were plotted to evaluate the model performance. A total of 792 patients with lumbar compression fractures were included in the study, and 188 patients had chronic postoperative low back pain, with an incidence of 23.74%. After univariate and multivariate logistic regression analysis, a total of 5 variables were identified as independent risk factors for chronic low back pain after undergoing PKP for lumbar compression fractures, namely having diabetes (OR, 1.607; 95% CI, 1.157–3.205), preoperative T value < −2.5 SD (OR, 2.697; 95% CI, 1.417–5.021), multiple lumbar fractures (OR, 1.815; 95% CI, 1.415–3.201), lumbar compression ≥ 50% (OR, 2.854; 95% CI, 1.411–6.524), and bone cement leakage (OR, 2.911; 95% CI, 1.715–6.817). The nomogram for chronic low back pain after PKP for lumbar compression fractures constructed in this study has good predictive accuracy and helps orthopedic surgeons to intervene earlier in patients at high risk of chronic low back pain after undergoing PKP for lumbar compression fractures.


Introduction
[3][4] Lumbar compression fractures are currently one of the most common lumbar spine disorders in the elderly population. [5,6][9] Compared with the first 2 treatment modalities, PKP has the advantages of less trauma, shorter operative time, less intraoperative bleeding, faster recovery, earlier ambulation, and fewer postoperative complications, etc. [10,11] PKP is currently the most mainstream modality for the treatment of lumbar compression fractures. [12]Although there are few complications after PKP, some studies have found postoperative re-fracture of the operated vertebral body and leakage of intraoperative bone cement, [13,14] but there is a lack of studies on chronic low back pain after fracture.Nomogram has been used in many fields, not only for predicting deep vein thrombosis in the lower extremities after fracture [15] but also for predicting post-fracture pneumonia [16] and post-fracture delirium, [17] with good predictive results.We aimed to investigate the independent risk factors for chronic low back pain after lumbar compression fracture undergoing PKP and to develop a nomogram prediction model accordingly.

Data sources
This study retrospectively analyzed the data of inpatient surgical lumbar spine compression patients from January 2017 to The authors have no funding and conflicts of interest to disclose.
The datasets generated during and/or analyzed during the current study are not publicly available, but are available from the corresponding author on reasonable request.
Due to the non-experimental nature of the research, the study protocol did not need to be submitted for consideration and approval to an ethical review committee.

Clinical outcomes and definitions
Patients were followed up 6 months after surgery in the outpatient clinic or by telephone, and those with a score > 4 according to the visual analog scale (VAS) were included in the observation group and those with a VAS score ≤ 4 were included in the control group.

Statistical analysis
The collected data were randomly divided into a training set (70%) and a validation set (30%) according to the ratio of 7:3 in R (4.2.1) software.The differences between observation and control groups were analyzed univariately in the training set using SPSS 26.0 software, and the chi-square test was used to statistically analyze the count data.Variables screened at P < .05from the univariate analysis were included in the multivariate logistic regression analysis and variables at P < .05from the multivariate logistic regression analysis were identified as independent risk factors for chronic low back pain after receiving PKP for lumbar compression fractures.The screened independent risk factors were plotted in a nomogram in R software, receiver operating characteristic (ROC) curves were plotted and the area under the curve (AUC) was calculated in the training and validation sets, and calibration curves and decision curves analysis (DCA) were plotted to assess model performance.

General information
A total of 792 patients with lumbar compression fractures were included in this study, and a total of 188 fracture patients had postoperative chronic low back pain, with an incidence of 23.74% of postoperative chronic low back pain.According to the ratio of 7:3, 556, and 236 fracture patients were randomly divided into training and validation sets.

Nomogram development and validation
A nomogram was drawn using the screened independent risk factors used to predict the risk of chronic low back pain after receiving PKP for lumbar compression fractures (Fig. 1).ROC curves were then plotted for the training and validation sets, and the corresponding AUCs were calculated to be 0.891 and 0.909 (Fig. 2A and B).In addition, calibration curves were plotted, indicating that the nomogram-predicted risk agreed well with the actual risk of occurrence and had the good predictive ability (Fig. 2C and D).Also, DCA showed that the nomogram had a good predictive ability (Fig. 2E and F).

Discussion
Although PKP is currently recognized clinically as the safest and most effective method for treating lumbar compression fractures with a low rate of postoperative complications, [10] some patients still have pain in the low back area after PKP treatment, which can have a serious impact on their lives.The current study collected clinical data from patients with lumbar compression fractures over the past 5 years to develop a clinical prediction model for determining the risk of developing chronic low back pain after PKP in such patients.In this study, we used predictors that are common and easily identified in clinical practice and developed this nomogram prediction model based on the 5 independent risk factors identified, and model validation determined that the model developed in this study has good predictive power.
The results of the study showed that having diabetes is a risk factor for chronic low back pain after receiving PKP for lumbar compression fractures.Studies [18,19] have shown that diabetes affects the vascular permeability of patients, causing peripheral microangiopathy, which decreases blood flow to the bone, affecting bone production and bone quality leading to increased fragility.At the same time, the microangiopathy of diabetes can cause changes in bone quality, resulting in brittle bones and fractures. [20]Diabetic patients have inherently unstable blood glucose levels, which can have a very significant impact on the patient postoperative recovery.
The results of the study showed that patient with osteoporosis is a risk factor for chronic low back pain after receiving PKP for lumbar compression fractures.Studies [21][22][23][24] showed that the greater the degree of vertebral osteoporosis, the lower the bone density values, and the more likely the vertebral trabeculae are to microfracture under stress, thereby stimulating the nerve endings and causing pain.Since osteoporotic microfractures can cause skeletal pain in patients, safe and effective anti-osteoporotic treatment should be administered to patients after surgery, which in turn relieves pain in the low back area.
The findings show that lumbar multiple lumbar fractures are a risk factor for chronic low back pain after receiving PKP for lumbar compression fractures.The lumbar spine carries all the weight, and an intact lumbar spine structure is the basis for function.In patients with multiple lumbar fractures, the instability of the lumbar spine increases and is less stable compared to a single lumbar fracture, which can only be partially stabilized most of the time with minimally invasive treatment and cannot maintain full stability. [25,26]Postoperatively, patients often experience decreased lumbar spine stability and chronic pain in the lumbar region.For multiple lumbar fractures, traditional open surgery treatment is often recommended, as traditional open surgery treatment is more stable for stabilizing multiple lumbar fractures. [27]ble 2 Multifactorial analysis of chronic low back pain after PKP.The findings show that lumbar compression above 50% is a risk factor for chronic low back pain after receiving PKP for lumbar compression fractures.The anterior column of the vertebral body is the main bearer of axial stress in the spine and is the most vulnerable part of the entire spine to collapse.[30] In patients with lumbar fractures, the height of the vertebral body has to be restored as much as possible when using bone cement filling.For vertebrae with high compression rates, the amount of bone cement injected should be increased.

危险因素
The findings show that intraoperative bone cement leakage is a risk factor for chronic low back pain after receiving PKP for lumbar compression fractures.[33] However, bone cement leakage may occur when cement filling is performed, and the leaking bone cement may irritate the surrounding nerves, resulting in back pain in patients.Therefore, when performing bone cement infusion, the speed of bone cement infusion should be controlled, and the amount of bone cement should be controlled.
However, there are shortcomings in this study.First, because this was a retrospective study, some unavoidable error arises.Second, this is a single-center study conducted at a tertiary referral trauma center, and there is bias in the selection of patients due to more severe fractures or more complex conditions in the admitted patients.Third, this is a risk prediction model developed in a single center, and therefore its validity needs to be validated in further multicenter studies.

Conclusion
The nomogram for chronic low back pain after PKP for lumbar compression fractures constructed in this study has good predictive accuracy and helps orthopedic surgeons to intervene earlier in patients at high risk of chronic low back pain after undergoing PKP for lumbar compression fractures.

Figure 1 .
Figure 1.Nomogram for predicting the risk of chronic low back pain after PKP.PKP = percutaneous kyphoplasty.

Table 1
Univariate analysis of chronic low back pain after PKP.
BMI = body mass index.